The present invention relates to a temperature stabilised ceramic material which has low dielectric loss and high thermal conductivity.
In current microwave communication technology dielectric resonators (DRs) are key elements for filters, low phase noise oscillators and frequency standards. DRs possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, can possess low temperature coefficients, high mechanical stability and small size.
Ceramic dielectric materials are used to form DRs as key components in a number of microwave subsystems which are used in a range of consumer and commercial market products. These products range from Satellite TV receiver modules (frequency converter for Low Noise Broadcast (LNB)), Cellular Telephones, PCN""S. (Personal Communication Networks Systems) and VSAT (Very Small Aperture Satellite) systems for commercial application to emerging uses in transportation and automobile projects, such as sensors in traffic management schemes and vehicle anti-collision devices. Dielectric Resonators may be used to determine and stabilise the frequency of a microwave oscillator or as a resonant element in a microwave filter. New systems of satellite TV transmission based on digital encoding and compression of the video signals determine the need for improved DR components. The availability of advanced materials will also enable necessary advances in the performance of DRs used for other purposes as referred to above.
A review on dielectric resonators and materials is given by Wersing in xe2x80x9cHigh Frequency Ceramic Dielectrics and their Application For Microwave Componentsxe2x80x9d in xe2x80x9cElectronic Ceramicsxe2x80x9d Ed BCH Steele Elsevier 67-119.
There are three key properties for a dielectric resonator. The first is the dielectric loss, or tanxcex4. The Q of the resonator (which in the absence of other losses may be approximated to 1/tanxcex4) determines the steepness of the filter skirts, the power requirements and the selectivity. The second is the dielectric constant (∈t). This determines the size of the resonator. Finally, the TCf, the temperature coefficient of the resonant frequency must be as near zero as possible. For high power filters it is very desirable for the dielectric to possess a high thermal conductivity, preferably greater than 20 W/m.K.
A ceramic material for use in dielectric resonators should have a low dielectric loss (high Q), a high dielectric constant, a TCf near zero and a high thermal conductivity.
However it is difficult to obtain a material with all these properties and in Table 1 below the properties of polycrystalline dielectric materials are given
The materials which are currently used as DR materials are those with TCf close to zero. In order to achieve low TCf in dielectric resonator materials the chemical composition of the material is altered. In most temperature stabilised ceramic compositions, the ceramic composition is substantially single phase e.g. Barium magnesium tantalate, barium zinc tantalate or zirconium tin titanate which are listed in Table 1. Whilst this is a very desirable approach, it is often the case that altering the chemical composition in order to achieve temperature stability causes a degradation in either the Q or the dielectric constant or both. Additionally, altering the chemical composition usually means that a second phase may be formed with undesirable TCf. For example, TiO2 in combination with Ba forms Ba2TiO9 which has an acceptable TCf of only 2 ppm/Kxe2x80x2 but an inferior dielectric constant of 40 and a Q of 15,000 at 2 GHz . There are certain ceramic composites in which there exist two separate phases of opposite TCf. For example Bi2O3xe2x80x94TiO2 composites in which a Bi2O11 phase is formed which possesses a TCf of xe2x88x92533 ppm/K, in opposition to the TiO2 phase which possesses a TCf of +450 ppm/K. However, the Q factor is rather poor at Q=1800 at 5 GHz and with a TCf of 21 ppm/K.
In general it is well known that ceramic materials possess very poor thermal conductivity. Notable exceptions are beryllium oxide 270 W/mK, silicon carbide 67 W/m.K, aluminium nitride 15 W/m.K. However, none of these materials possesses particularly low dielectric loss.
Another approach is to use a composite dielectric resonator with improved frequency stability using two different zirconate materials with opposite TCf characteristics. A disadvantage of these materials is that the thermal conductivity of the zirconate materials is less than 4 W/m.K. Temperature compensated whispering gallery mode resonators have been examined for use at cryogenic temperatures. These use a single crystal sapphire disc and sandwiched the sapphire with thin rutile or strontium titanate plates. In the case of rutile plates, high Q and temperature stability was achieved at temperatures between 50-160K. The use of strontium titanate plates, although achieving temperature compensation, considerably reduced the Q of the composite due to the high dielectric loss of the strontium titanate. The problems with this approach are the cost of the single crystals and the fact that attaching the single crystals effectively is not trivial.
Other attempts to obtain dielectric materials with improved properties are disclosed in U.S. Pat. Nos. 391,672, 3,798,578, 4,580,116, 562,556 and 5,909,160.
Aluminium oxide is a well known ceramic dielectric material and has a room temperature dielectric constant of approximately 10 and previous works has shown that extremely low tanxcex4 can be achieved (tanxcex4=2xc3x9710xe2x88x925, 300K 10 GHZ). The thermal conductivity of alumina is moderately high around 20-30 W/m.K at room temperature and this is a great advantage in high power filters operating at around 100 W.
Alumina has a TCf of xe2x88x9260 ppm/K and it would be highly desirable to be able to tune the TCf while maintaining a low tanxcex4.
We have now devised an alumina based ceramic material with good TCf and low tanxcex4.
According to the invention there is provided a ceramic composition which comprises an alumina sintered body on which there is a layer of titanium dioxide.
In this specification ceramic means any solid inorganic particulate material, the particles of which can be caused to sinter together by the application of heat.
The alumina is preferably doped with from to 0.05 to 0.5 wt. % of the titanium dioxide and more preferably with from 0.01 to 1.0 wt. % of the titanium dioxide. The compositions of the present invention can be made by mixing alumina powder with titanium dioxide powder and compressing and sintering the mixed powder using titanium dioxide powder and compressing and sintering the mixed powder using conventional methods to obtain a doped alumina ceramic material. It is not a requirement for the present invention that the alumina ceramic be doped with TiO2. The alumina sintered body or in the preferred form, the Ti doped alumina body will hereinafter be referred to as alumina in order to avoid confusion.
The alumina should contain the minimum of impurities as impurities can adversely affect the dielectric properties of the alumina.
The layer of the titanium dioxide preferably comprises a volume fraction between 0.0001 to 0.5 and more preferably between 0.001 to 0.05.
The alumina and titanium dioxide layer should be in intimate contact and preferably they are attached by solid state diffusion.
The titanium oxide layer can be formed on the alumina by forming a paste of the titanium dioxide and applying the paste to the alumina, drying and heating to form a dense layer of titanium dioxide on the doped alumina.
The paste of the titanium dioxide is preferably made with non-aqueous liquids such as polymers and non-aqueous solvents, the nature of the liquid is not critical.
The temperature of heating should be sufficient to densify the titanium dioxide and for example is preferably at least 1000xc2x0 C.
The invention also provides a layered composite dielectric in which the two compositions, the alumina and the titanium dioxide layer, are attached by solid state diffusion and which has a very high Q a high thermal conductivity and a low temperature coefficient of the resonant frequency over a very wide temperature range.
The invention is described in the following Examples in which high purity alumina powder was doped with TiO2 at the 0.2 wt. level. (as described above, doping with TiO2 has been demonstrated to reduce the dielectric loss in sintered alumina considerably but it is not a necessity if a suitably high Q can be obtained in the alumina without doping with TiO2.) The powder was pressed at 100 MPa in a 13 mm stainless steel die set. The discs were sintered in air in a muffle furnace at a ramp rate of 5xc2x0 C.minxe2x88x921 to 1600xc2x0 C., dwell for 1 hour and 10xc2x0 C.minxe2x88x921 to room temperature. The discs were weighed. The unloaded Q of 27 samples was determined at a frequency of approximately 10 GHZ and found to be Q=54,772, 1 standard deviation=2714, coefficient of variation=4.9%.
A thick film paste of TiO2 was prepared with high purity TiO2 powder. The powder was thoroughly mixed with a vehicle comprising non-aqueous polymers and solvents on a Marchant 3 roll mill. A special jig was designed in which different thickness of shim ranging from 100 xcexcm to 300 xcexcm could be placed on top of the sintered alumina disc. This enabled the manufacture of a range of depths for the TiO2 coating. The thick film paste was applied on to the surface of the alumina disc. The paste was dried at 80xc2x0 C. and then the composite was fired at 1400xc2x0 C. for 1 hour. This was sufficient to densify the TiO2. The result was a dense TiO2 layer on a dense alumina disc. The composite disc was then weighed and the weight of TiO2 was determined by difference. The volume fraction of the TiO2 and A1203 was determined by noting that the density for A1203 is 3.9/Mg mxe2x88x923 and that for TiO2 is 4.26 Mg mxe2x88x923.
The unloaded Q was measured at approximately 10 GHz by a resonant cavity method using the TE01xcex4 mode. The sample was placed in an oxygen-free high conductivity copper cavity on a 4 mm high low loss quartz spacer. The cavity was 30 mm in diameter, with adjustable height. The surface resistance of the copper was calculated from the Q of the TE011 resonance of the empty cavity to allow the results to be corrected for the loss due to the cavity walls. The TE01xcex4 and mode was examined using a vector network analyser (Hewlett Packard HP8720C) with 1 Hz resolution. Particular care was taken to ensure that the samples were dry. On removal from the furnace, samples were placed in a vacuum desiccator over silica gel, using plastic tweezers to minimise surface contamination; when cool, samples were placed in the microwave cavity for measurement.
The TCf measurements were performed over a temperature range of 150K-320K. The sample was placed on the floor of a copper cavity identical to that described above but without a quartz spacer. Cooling and heating the cavity was achieved by placing the cavity on the cold-head of a closed cycle Gifford McMahon two stage cryocooler (CTI) capable of a temperature range 320K-10K.
The results are shown below, The turning point temperature Tp is defined as that temperature where the TCf is zero